Growth and water use dynamics of New Zealand indigenous and exotic tree species in the Wairarapa – a case study

Abstract

Little is known about the effect of New Zealand indigenous tree species on the forest hydrological cycle and there are limited quantitative growth and transpiration data for New Zealand indigenous tree species, particularly in a plantation setting. This thesis aims to compare growth and transpiration responses of indigenous and exotic tree species to weather conditions in a plantation setting over a two-year study period (Year 1 = 21/06/2022-21/06/2023, Year 2 = 22/06/2023-21/06/2024). This thesis also aims to assess the suitability of electronic point dendrometers as alternatives to the widely used but expensive and invasive sap flow meters for estimating tree and forest level transpiration. To achieve our aims, eight study species were selected for this thesis including three indigenous Nothofagaceae species, Fuscospora fusca, F. solandri, and Lophozonia menziesii, two indigenous Podocarpaceae species, Podocarpus totara and Prumnopitys taxifolia, one exotic angiosperm species, Eucalyptus fastigata (Myrtaceae), and two exotic conifer species, Pinus radiata (Pinaceae) and Sequoia sempervirens (Cupressaceae). Rewanui Forest Park (Wairarapa region) was selected for this thesis as it is a rare example of a forestry trial including both indigenous and exotic species. Electronic point dendrometer measurements revealed that only one of the exotic study species, S. sempervirens, had a faster growth rate than the native species during the study period, despite the much larger size of the exotic species. The other two exotic species, E. fastigata and P. radiata, were limited by canopy closure and wet microsite conditions respectively. Modelling growing season duration using the Gompertz equation highlighted the role of species-traits and the impact of weather conditions on growth. For example, F. fusca and L. menziesii demonstrated high instantaneous growth rates but a short growing season duration, particularly under drier weather conditions, suggesting that these were the most climate sensitive of the study species. Notably, all study species were modelled to cease growing on the last day of Year 1 which was wetter than Year 2. This result suggests that all study species were able to continue growing at a rate within the model parameters until the winter solstice (21st June 2023), likely due to higher than normal rainfall in late summer and early autumn of Year 1. Differences in weather also affected the other dendrometer variables. Notably, one of the shrinkage variables, tree water deficit (TWD), was low in Year 1 but exhibited a large summer peak in Year 2 for all study species. Microsite conditions appeared to affect TWD. For example, the high TWD peak for E. fastigata was likely driven by dry microsite conditions, large tree size and high sap flux densities causing increased soil water depletion and stem shrinkage in this species. The large size and likely high leaf area of E. fastigata also appeared to drive higher rates of sap flux density compared to the indigenous species, as revealed by heat ratio method sap flow meter measurements. Larger stem sizes appeared to drive higher rates of total water use for all three of the exotic species compared to the indigenous species. The effects of weather and climate on sap flux density were generally species specific. For example, little sap flux density variation was explained by the climate variables for P. taxifolia, which consistently ranked low for all measured and calculated variables. Maximum daily shrinkage (MDS), emerged as the best overall dendrometer variable for predicting sap flux density, with higher correlation and prediction strength for most of the study species. However, under the drier conditions of Year 2, TWD emerged as a stronger predictor for some of the indigenous species, namely, F. solandri, L. menziesii, P. totara, and P. taxifolia. Most of these species were, in some way, restricted by soil water conditions. Fuscospora solandri had low soil volumetric water content at the study site, L. menziesii is considered to be naturally adapted to wetter conditions than at our study site, and P. totara had high competition for soil water due to high planting density at the study site. Including climate variables helped to improve prediction strength of the dendrometer variables across all species. Together these results highlight the role of microsite conditions, stem size, leaf area, and species-traits on the impacts of trees on components of the forest hydrological cycle such as transpiration. These results also highlight the importance of species selection for afforestation projects. This work advances our understanding of water use and growth dynamics of New Zealand indigenous species that have been previously understudied, and how those dynamics compare to exotic plantation tree species.